As per ACI 318 cl. 21.3.2.2 "the positive moment strength at joint face shall be not less than one-half of the negative moment strength provided at that face of the joint. Neither the negative nor the positive moment strength at any section along member length shall be less than one-fourth the maximum moment strength provided at face of either joint."( ref ACI 318).My query is whether this clause is applicable for cantilever type of pier cap/pier heads with single pier?

I think as there no possibility of reversal of forces even in case of seismic or any other load case, the minimum 12 mm dia bars are sufficient for these kind of structures.